v3.26.1
Cover - shares
 3 Months Ended
Mar. 31, 2026
May 11, 2026
Document Information [Line Items]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Mar. 31, 2026  
Document Transition Report false  
Entity File Number 001-37621  
Entity Registrant Name FIRST GUARANTY BANCSHARES, INC.  
Entity Incorporation, State or Country Code LA  
Entity Tax Identification Number 26-0513559  
Entity Address, Address Line One 400 East Thomas Street  
Entity Address, City or Town Hammond,  
Entity Address, State or Province LA  
Entity Address, Postal Zip Code 70401  
City Area Code (985)  
Local Phone Number 345-7685  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Non-accelerated Filer  
Entity Small Business true  
Entity Emerging Growth Company false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   16,464,248
Entity Central Index Key 0001408534  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2026  
Document Fiscal Period Focus Q1  
Amendment Flag false  
Common Stock    
Document Information [Line Items]    
Title of 12(b) Security Common Stock, $1 par value  
Trading Symbol FGBI  
Security Exchange Name NASDAQ  
Noncumulative Preferred Stock    
Document Information [Line Items]    
Title of 12(b) Security Depository Shares (each representing a 1/40th interest in a share of 6.75% Series A Fixed-Rate Non-Cumulative perpetual preferred stock)  
Trading Symbol FGBIP  
Security Exchange Name NASDAQ